In recent years, with the advancement of digital technology, portable digital apparatuses such as small and flat digital AV players and digital cameras have been developed to provide higher functionality. A demand for large-capacity and high-speed nonvolatile memory devices used as memory devices in these apparatuses is increasing more and more. To meet such a demand, nonvolatile memory devices using a variable resistance element or a ferroelectric capacitor, a type of nonvolatile memory element, have attracted attention.
Variable resistance elements are classified into a write-once type and a rewritable type. The rewritable variable resistance elements are further classified into two types. One of the two types is variable resistance elements having characteristics of changing from a high resistance state to a low resistance state and vice versa with two driving voltages having the same polarity. These variable resistance elements are generally referred to as unipolar (or monopolar) variable resistance elements. The other of the two types is variable resistance elements having characteristics of changing from a high resistance state to a low resistance state and vice versa with two programming voltages having different polarities. These variable resistance elements are generally referred to as bipolar variable resistance element.
In a nonvolatile memory device having variable resistance elements arranged in an array, current steering elements such as transistors and rectifying devices are generally connected in series with the variable resistance elements. With this, write disturb caused by bypass current in the array, cross talk between memory cells adjacent to each other, and so on are prevented, and more reliable memory operation is performed.
The unipolar variable resistance elements are capable of controlling a resistance change operation with two different programming voltages having the same polarity. For this reason, a unidirectional diode which uses only the nonlinear voltage-current characteristics in one of polarities of a voltage can be used for a diode as a current steering element. Thus, there is a possibility of simplifying a structure of a memory cell including a variable resistance element and a current steering element. However, because a reset operation to change the variable resistance element to a high resistance state requires an electric pulse having a wide pulse width, the unipolar variable resistance elements operate slowly.
In contrast, the bipolar variable resistance elements are capable for controlling resistance change with two programming voltages having different polarities. For this reason, a bidirectional diode which uses nonlinear voltage-current characteristics in the both polarities of the voltages is required for a diode as a current steering element. However, because both the reset operation to change a variable resistance element to a high resistance state and a set operation to change the variable resistance element to a low resistance state can be performed using an electric pulse having a narrow pulse width, the bipolar variable resistance elements are capable of operating fast.
The cross point nonvolatile memory devices as described in Patent Literatures (PTLs) 1 and 2 have been proposed so far.
The nonvolatile memory device described in PTL 1 includes memory cells in each of which a unidirectional diode is connected as a current steering element in series with a variable resistance element. Here, the unidirectional diode is a PN junction diode or a Schottky diode, for instance.
The nonvolatile memory device described in PTL 2 includes memory cells in each of which a bidirectional diode is connected as a current steering element in series with a variable resistance element.
For example, a metal-insulator-metal (MIM) diode, a metal-semiconductor-metal (MSM) diode, and a varistor as described in PTL 2 are known as the bidirectional diode.
FIG. 27 is a graph showing voltage-current characteristics of a commonly-known bidirectional diode. Such voltage-current characteristics are observed in the bidirectional diode such as the MIM diode, the MSM diode, and the varistor.
In these bidirectional diodes, by optimizing electrode materials and materials interposed between electrodes, it is possible to make the voltage-current characteristics substantially symmetrical with respect to a polarity of an applied voltage. In other words, it is possible to achieve characteristics that a change of current relative to a positive applied voltage and a change of current relative to a negative applied voltage are made substantially symmetrical about the origin 0.
Moreover, as shown in FIG. 27, electrical resistance of the bidirectional diode is very high when the applied voltage is less than or equal to the first critical voltage Vth1 and more than or equal to the second critical voltage Vth2 (range C in FIG. 27), and the electrical resistance of the same rapidly decreases when the applied voltage exceeds the first critical voltage Vth1 or falls below the second critical voltage Vth2 (ranges A and B in FIG. 27).
By combining bidirectional diodes having such voltage-current characteristics and bipolar memory elements, that is, using the bidirectional diodes as the current steering elements, it is possible to achieve a cross point nonvolatile memory device using bipolar variable resistance elements.